As disclosed in U.S. Pat. No. 5,445,498, for example, there is known a multi-arc blade in which a plurality of arcs and straight lines are connected to each other such that only a gradient is continuous at respective junctions between adjacent two of those arcs and straight lines. As represented by such a multi-arc blade, the profile of a known turbine blade has not been designed so as to keep continuity in the curvature of a blade surface from a leading edge to a trailing edge thereof. The multi-arc blade is relatively easy to design and manufacture, but it is disadvantageous in that a pressure distribution along the blade surface is distorted at points where the curvature is discontinuous and a surface boundary layer is thickened with the distortion, thus resulting in a larger profile loss.
Regarding other known turbine blade than the multi-arc blade, JP,A 6-1014106, for example, discloses a design method comprising the steps of arranging arcs along a camber line of a blade and forming a profile of the blade as a circumscribed curve with respect to a group of those arcs. According to that design method, a leading edge and a trailing edge are each formed in an arc shape, but the curvature is discontinuous at junctions between those arc-shaped portions and other adjacent portions forming the blade profile. Hence, the curvature of the blade leading edge is extremely large, while the curvature of the blade surface is reduced in a portion just downstream of the blade leading edge. For that reason, if an inflow angle differs from the design setting point of the blade, a boundary layer is thickened or peeled off at the point where the curvature is discontinuous, thus causing a profile loss.
Further, in an area where a curvature distribution along the blade surface increases or decreases from the upstream toward downstream side, the blade surface pressure is reduced at a maximum point of the curvature, and an inverse pressure gradient occurs downstream of that point. Therefore, a boundary layer is thickened or peeled off, thus resulting in a larger profile loss.
Moreover, U.S. Pat. No. 4,211,516, for example, discloses a blade profile in which a trailing-edge wedge angle formed by a suction surface near a blade trailing edge and a tangential line with respect to a pressure surface is as large as about 10 degrees. In such a blade profile, a fluid flowing along the blade suction surface and a fluid flowing along the blade pressure surface collide against each other at the trailing edge, thus resulting in a larger profile loss.
An object of the present invention is to provide a turbine blade capable of reducing the profile loss.